In recent years, high purity hydrogen gas has drawn attention as a fuel gas used in energy systems such as hydrogen fuel cells and hydrogen gas turbines. It is known that the high purity hydrogen gas is produced from a hydrogen-containing source gas such as a mixed gas obtained by electrolyzing water or a mixed gas obtained by steam reforming liquefied natural gas (LNG) due to the following process by employing a high-performance hydrogen purifier like that shown in the schematic diagram in FIG. 5. The purifier is partitioned into a left-hand chamber and a right-hand chamber by a hydrogen permeation/separation membrane, which is made of a material permeable only to hydrogen and which has a thickness of 0.1 to 3 mm, and is reinforced at the periphery with a frame body made of nickel or the like. A hydrogen-containing source gas inlet tube and an exhaust gas outlet tube are installed in the left-hand chamber, whereas a high purity hydrogen gas outlet tube is installed in the right-hand chamber. A reaction chamber made of a material such as stainless steel is provided at the center of the purifier. The reaction chamber is heated to 200 to 300° C. and the hydrogen-containing source gas is introduced from the inlet tube. While maintaining the internal pressure of the right-hand chamber where the hydrogen separated/purified by the hydrogen permeation/separation membrane is present at 0.1 MPa and the internal pressure of the left-hand chamber where the hydrogen-containing source gas is present at 0.2 to 0.5 MPa, the high purity hydrogen gas is produced by a separation/purification process due to the hydrogen permeation/separation membrane.
In addition, the wide use of the abovementioned hydrogen permeation/separation membrane in the chemical reaction processes including the steam reforming process of hydrocarbons and the hydrogenation/dehydrogenation processes such as the reaction between benzene and cyclohexane where hydrogen is selectively transferred is also well known.
Moreover, it is also known that the abovementioned hydrogen permeation/separation membrane is constituted from an Ni—Ti—Nb alloy having the following composition (α) and alloy structure (β):    (α) a composition consisting of 25 to 45 atomic % of Ni, 26 to 50 atomic % of Ti, and a remainder containing Nb and inevitable impurities (with the proviso that the Ni content is 11 to 48 atomic %); and    (β) with respect to a cast thin plate cut out from a cast ingot by electrical discharge machining and having a thickness of 0.1 to 3 mm, an alloy structure which has a eutectic microstructure of a solid solution of Ni in an NbTi phase and a solid solution of Nb in an NiTi phase, and also has a primary NbTi phase (white islands seen in FIG. 4) dispersed in the microstructure as shown in the photographs of structures in FIGS. 2 and 4 taken by a scanning electron microscope (magnification: 2500× in FIGS. 2 and 4000× in FIG. 4).
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-232491
The demands for various chemical reactors with higher performance including the above high-performance hydrogen purifier are extremely high. Accordingly, even higher performance in the hydrogen permeation/separation is required for the hydrogen permeation/separation membrane used as a structural member in the aforementioned reactors. In addition, when the aforementioned hydrogen permeation/separation membrane is used, since it is known that its hydrogen permeation/separation performance generally improves as its membrane thickness is reduced, studies concerning the development of highly strong Ni—Ti—Nb alloy that constitutes the aforementioned hydrogen permeation/separation membrane have been conducted intensively. However, since the Ni—Ti—Nb alloy that constitutes the conventional hydrogen permeation/separation membranes has insufficient mechanical strength, the thickness of the membrane could not be reduced to 0.1 mm or less, and thus satisfactory improvement in the hydrogen permeation/separation performance has currently not been achieved.